This is just a thought from a never was dub.
WT (and Mormons) could double their coverage if they really wanted to by sending out their people on their own (just as, here, door knockers selling telecoms packages, chugging for charities, etc., do).
I assume (being cynical) that the reason they go in pairs is just in case a dub on FS meets someone who could talk to them and challenge their beliefs. The 'partner' dub is there to prevent that. If my assumption is correct, that says someting about the dubs - about their knowledge and confidence in their faith.